1. Technical Field
The present invention relates to a method of delivering and retaining borate in a wet process hardboard substrate useful for construction applications such as residential and commercial wood-frame construction. More particularly, the present invention relates to a method of impregnating a wet process hardboard with zinc borate to create a product resistant to long-term decay caused by the penetration or infiltration of water and fungus as well as damage caused by termites and other insects. The invention also includes the hardboard containing zinc borate produced by the novel process.
2. Background Art
Zinc borate is a known and established wood preservative for enhancing the resistance of wood and wood composite compounds against natural environmental stresses. Zinc borate has a nominal chemical formula of 2ZnO-3B2O3-3.5H2O and a median particle size of 7 microns. Borates are generally non-toxic to humans, mammals, and most aquatics at low to moderate concentration levels, but they do impede the metabolism of soil or wood-borne organisms such as fungi, mold, and some bacteria. Additionally, borates have been proven to be toxic to many insects, most notably termites. Thus, borates have experienced long use in the wood manufacturing process dating from the mid-1900's as one of the very first wood preservatives.
Borates can be introduced to wood as either boric acid or as sodium borate, calcium borate, or zinc borate. The borate ion itself imparts the predominant fungal, rot, and insect resistant qualities to the wood substrate. However, because the borate ion is highly soluble, it requires a generally inert and stabilizing chemical carrier to which it can bond and become electrochemically neutral. To achieve this electrochemical neutrality and thus render the borate ion virtually insoluble, borates are often reacted with sodium, calcium, or zinc (e.g. cations with a valence charge of +2). Sodium borate has been the most common borate compound used for wood preserving. However, zinc borate has gained popularity and today is becoming one of the more popular wood preservatives due to the zinc portion of the zinc borate chemistry having some level of water, fungal, and insect-resistant properties. As a result, the exterior wood manufacturing and/or wood composite industry has recognized zinc borate as one of the best overall wood preserving compounds.
Traditionally, the use of borate compounds as wood preservatives has been exclusively for treated lumber or I-joist manufacturing and dry process wood composite manufacturing such as Oriented Strand Board (OSB) or Medium Density Fiberboard (MDF). In the case of treated lumber or I-joists, sodium borate is typically used and is applied via a direct aqueous spray process. In the case of the dry processing of wood composites in board mills such as OSB or MDF, zinc borate is introduced to the process at a blending operation. Physically, zinc borate is a stable, white, high molecular weight, crystalline powder that when mixed with water, generally does not dissolve. Zinc borate is initially formulated from zinc oxide, boric acid, and water. Furthermore, even in the presence of water at a neutral pH, the zinc borate does not typically dissociate or experience phase or chemical change. However, if the pH of the zinc borate and dry wood article were to become considerably acidic or alkaline, for instance experience a pH of 3.0 or lower, or conversely 9.0 or higher, the zinc borate would become progressively more soluble and begin to chemically change to zinc hydroxide (or oxide) and boric acid. This dissolved product of zinc and boric acid would no longer possess the favorable properties of zinc borate and therefore have a substantially reduced value as a wood preservative.
Zinc borate can and has been used with great success in dry process wood composite board mills where the wood's interior moisture content is considerably low (e.g. typically <10%) because the borate can bond to the phenol formaldehyde (PF) or methyl diphenyl isocyanate (MDI) resins through both physical and chemical bonding. Conversely, in wet process, composite wood manufacturing environment such as hardboard, specifically exterior hardboard siding, the ability to introduce and retain borate compounds has been met with virtually no success. Because borates can be highly soluble in water, the borates would simply convert to dilute boric acid solutions and be lost during the traditionally low pH and high temperature wet forming and/or wet-pressing processes.
Much research has been conducted on creating different types of preservatives as well as methods of making preserved dry board. For example, U.S. Pat. No. 4,871,473 by Goettsche et al., discloses a wood preservative based upon a zinc compound in aliphatic carboxylic acid which also contains a polyamine for creating a preservative for wood which does not cause discoloration. Furthermore, this disclosed compound is stable and the solution does not precipitate and can be applied by spraying, dipping, or impregnating the wood, or also by painting it upon the wood.
In Knudson et al. (U.S. Pat. No. 4,879,083), a method is disclosed for making particle board impregnated with a borate compound. Specifically, the method includes treating the wood particles with phenol formaldehyde resin and adding either anhydrous borax or zinc borate compound and then consolidating the treated particles under heat and pressure under normal processing conditions.
In Shiozawa, U.S. Pat. No. 5,478,598, a wood preservative and method for treating a wood is disclosed wherein the wood preservative compound contains a copper compound, a zinc compound, an aqueous salt, and a volatile basic compound. The wood preservative is alleged to be fungi resistant, insect resistant, fixed into the wood and also allegedly possesses a low toxicity.
In U.S. Pat. No. 5,972,266, Fookes et al. discloses a process for making a consolidated wood board containing zinc borate. This processes discloses first forming a sprayable aqueous emulsion of zinc borate and applying the emulsion to the wood strands prior to consolidating the wood strands together to form the composite wood material.
In Lehtinen et al. (U.S. Pat. No. 6,030,562), a method of forming a consolidated wood article is disclosed wherein zinc borate and a resin are mixed with either wood chips or wood fibers to form a wood board. The process further includes pressing the combination of zinc borate resin with the substrate material under heat supplied by steam to provide the wood composite article.
Laks et al. (U.S. Pat. No. 6,521,288) describes production of a wood product by incorporating a nanoparticle containing a biocide with wood particles and applying a sufficient pressure to the wood particles to form the wood product. It is suggested that this disclosed process is more desirable and less toxic than other methods.
In U.S. Pat. No. 6,881,247, Batdorf discloses a protective barrier coating for use upon wood. The disclosed coating includes a metal borate compound, a zinc compound, magnesium hydroxide, and a water-based binder for protection against insects, mold or mildew, and fire or water damage.
Despite the success of incorporating zinc borate into wood or wood composites during dry processing, zinc borate has been extremely ineffective in wood preservation of wood articles produced through wet processing. The zinc borate typically will be made soluble within the wet process wood article and thus will not impart the resistant characteristics to a hardboard produced through wet processing.
What is desired, therefore, is a method for effectively retaining borate in a wet process hardboard product, such as an exterior siding product, wherein the borate containing wet process hardboard meets or exceeds the minimum standards for wood composites set forth by the American Wood Preservers' Association, specifically 0.38% BAE for decay caused by basidiomycetes and 0.30% BAE for subterranean termite/insect resistance. The net result would be the manufacture of a product which is largely resistant to termitic degradation and fungal rot.